The document discusses various capabilities of Mule Expression Language (MEL) including: 1) How to copy and restore the message payload using set-variable and set-payload. 2) How to extract message attachments using a filtered projection. 3) How MEL supports regular expressions, XPath, and JSON processing. 4) How to include DataWeave code using dw() and split() functions to perform complex data transformations. 5) Miscellaneous operations like assigning variables, appending strings, calling static methods, and creating objects.

1. Mule Concepts
Mule Expression Language (MEL)
Basics -V

2. • Payload and Attachments
– To copy the current payload in a flow variable
named originalPayload then restore it :
<set-variable variableName="originalPayload"
value="#[message.payload]" /> <set-payload value="#[originalPayload]"
/>
– To retrieve the message payload in a particular format, using Mule’s
auto-transformation capability, use payloadAs:
<logger message="#[message.payloadAs(java.lang.String)]" />
– To extract all .txt and .xml attachments, use a filtered projection:
<expression-transformer expression="#[($.value in
message.inboundAttachments.entrySet() if $.key ~=
'(.*.txt|.*.xml)')]" />
– To ask for a null payload:
#[payload is NullPayload]
Mule Expression Language (MEL)

3. • Regex Support
– Regular expression helper functions retrieve null, a single value or an
array of values, depending on matches.
– The forms that take a melExpression argument apply the regex to the
result of its evaluation instead of message.payload.
#[regex(regularExpression [, melExpression [, matchFlags]])]
– Example :
• to select all the lines of the payload that begin with To:, From:, or Cc: use:
#[regex('^(To|From|Cc):')]
Mule Expression Language (MEL)

4. • XPath Support
– XPath helper functions return DOM4J nodes.
– By default the XPath expression is evaluated
onmessage.payload unless an xmlElement is specified:
#[xpath3(xPathExpression [, xmlElement])]
– To get the text content of an element or an attribute:
#[xpath3('//title').text]
#[xpath3('//title/@id').value]
Mule Expression Language (MEL)

5. • JSON Processing
– MEL has no direct support for JSON.
– The json-to-object-transformer can turn a JSON payload into a
hierarchy of simple data structures that are easily parsed with MEL.
– For the equivalent of this JSON path expression:
$..[? (@.title=='Moby Dick')].price
The following uses a filtered projection:
<json:json-to-object-transformer returnClass="java.lang.Object" />
<expression-transformer expression='#[($.price in message.payload if
$.title =='Moby Dick')[0]]" />
Mule Expression Language (MEL)

6. • Including DataWeave code
– You can carry out powerful complex data transformations by
including MEL DataWeave Functionsthat use DataWeave Language
code.
– You can include this code via two different functions in MEL: 'dw()' and
'split()'.
– 'dw' simply executes the DataWeave code you pass as an argument
and returns the transformation’s result
– 'split()' executes the code you pass as an argument and returns an
iterator that allows you to process each instance of the output as a
separate message.
dw("myobject:{id:payload.accountid, user:payload.user}")
Mule Expression Language (MEL)

7. • Miscellaneous Operations
– Assign to variable lastname the value of the message
inbound property lastname:
#[lastname = message.inboundProperties.lastname]
– Append a string to the message payload:
#[message.payload + 'mystring']
– Call a static method:
#[java.net.URLEncoder.encode()]
– Create a hash map:
#[new java.util.HashMap()]
Mule Expression Language (MEL)